The present invention relates to an in-cylinder injection type internal combustion engine, and more specifically, to a catalyst-temperature raising technique utilized in an in-cylinder injection type internal combustion engine.
Recently, in-cylinder injection type internal combustion engines, which inject fuel directly into a combustion chamber, have been put to a practical use. This type of an internal combustion engine generally undergoes such control as to switch between a stratified combustion and uniform combustion in accordance with operating conditions of the engine.
This type of internal combustion engine is also provided with an exhaust-gas purifying catalyst device in its exhaust passage to suppress harmful components or emissions in the exhaust gas emitted in the atmosphere. The temperature of this catalyst device should be raised to or above a predetermined temperature in order to acquire a sufficient exhaust-gas purifying characteristic. In the field of the aforementioned internal combustion engines, there have been developed a scheme of activating the catalyst device faster and a scheme of preventing the temperature of the catalyst device from falling below the predetermined temperature.
The technique disclosed in Japanese Unexamined Patent Publication (KOKAI) No. Hei 8-100633, for example, carries out a sub-injection in the first half of the power stroke or expansion stroke after executing a main injection for obtaining the output power and reburns the fuel in the sub-injection by utilizing the flame propagation of the main combustion from the main injection. This technique raises the temperature of the exhaust gas earlier to thereby quickly activate the catalyst device after cold starting.
There may be a case where a stratified combustion operation (e.g., an idling operation), in which the temperature of the exhaust gas becomes lower than that in a uniform combustion operation, continues after warm-up, thereby causing the temperature of the catalyst device to fall. In such a case, the technique disclosed in Japanese Unexamined Patent Publication (KOKAI) No. Hei 10-47040 keeps the catalyst temperature at or above a predetermined activation temperature by switching the operation of the in-cylinder injection type internal combustion engine to the uniform combustion operation from the stratified combustion operation. This prevents deterioration of the exhaust-gas purifying characteristic.
Because the former technique needs to inject a large amount of extra fuel, which does not contribute to the output of the engine, in order to increase the catalyst temperature, the fuel consumption gets higher. Since the amount of the temperature of the exhaust gas to be raised is not large in the latter technique, the time for the uniform premix combustion operation to keep or recover the temperature of the catalyst device becomes longer. This results in higher fuel consumption.
Accordingly, it is an object of the present invention to provide an in-cylinder injection type internal combustion engine which can improve the exhaust-gas purifying characteristic while suppressing an increase in fuel consumption.
To achieve this object, according to one aspect of this invention, an in-cylinder injection type internal combustion engine having fuel injection valves for injecting directly into combustion chambers of the internal combustion engine comprises, an exhaust-gas purifying catalyst device, provided in an exhaust passage of the internal combustion engine, for purifying harmful components in an exhaust gas; and a control device having a temperature-increase control section for achieving a stratified combustion in such a way that an air-fuel ratio of the internal combustion engine becomes a stoichiometric air-fuel ratio or in a vicinity thereof when an increase in a temperature of the exhaust-gas purifying catalyst device is needed.
As this structure achieves a stratified combustion in such a way that the air-fuel ratio of the engine becomes a stoichiometric air-fuel ratio or in a vicinity thereof, it is possible to feed plenty of carbon monoxide (CO) and oxygen (O2) into the exhaust-gas purifying catalyst device at the same time to cause an oxidation reaction without impairing the fuel consumption. This can allow the temperature of the exhaust-gas purifying catalyst device to be increased efficiently by reaction heat.
In this case, it is preferable that the control device performs open-loop control in such a manner that the air-fuel ratio becomes approximately 14 to 16.
The in-cylinder injection type internal combustion engine preferably further comprises an oxygen-concentration detecting device, provided in the exhaust passage, for detecting an oxygen concentration in the exhaust gas, whereby the control device performs a feedback control based on an output of the oxygen-concentration detecting device in such a way that the air-fuel ratio becomes the stoichiometric air-fuel ratio.
This can enhance reliability of the control and can improve the efficiency of raising the temperature of the exhaust-gas purifying catalyst device.
It is preferable that the in-cylinder injection type internal combustion engine further comprises ignition plugs provided on an engine body in such a way as to face the combustion chambers, and the control device further includes an injection-timing control section for controlling injection timings of the fuel injection valves and ignition-timing control section for controlling ignition timings of the ignition plugs, whereby at a time the temperature-increase control section is activated, the control device adjusts either each of the injection timings set by the injection-timing control section or each of the ignition timings set by the ignition-timing control section in such a way as to make an interval between those injection timing and ignition timing longer than that in stratified combustion in which the temperature-increase control section is not activated.
This structure can efficiently raise the temperature of the exhaust-gas purifying catalyst device while suppressing generation of smoke.
In the in-cylinder injection type internal combustion engine, it is preferable that the control device further includes an additional-fuel injection control section for injecting fuel in an injection other than a main injection for main combustion, whereby before activation of the temperature-increase control section, the control device causes the additional-fuel injection control section to inject additional fuel in an expansion stroke after the main injection.
This structure can improve the efficiency of raising the temperature of the exhaust-gas purifying catalyst device, particularly, at the time of cold starting.